To reset a passcode-protected Kindle, enter ‘ 111222777 ‘ as the passcode and tap OK. This applies to the 6th gen or later Kindle devices.

How to Reset a Locked Kindle Fire. If you are locked out of your Kindle Fire or Kindle Fire HD because you have forgotten your password, the only way to get back in is to perform a factory reset ...

If you ever forget your Kindle password, you must reset the e-book reader. The e-book reader doesn't include a method to reset just your Kindle's password.